Opportune moment to rediscover Chennai’s hydrology

After repeated flooding in the city, the key issue now is to decode Chennai’s urban and peri-urban hydrology, its ecosystem in its totality, and make meaningful and scientific interventions

Updated - December 16, 2023 09:14 am IST

Published - December 16, 2023 12:16 am IST

In north Chennai

In north Chennai | Photo Credit: The Hindu

Unusually heavy rainfall years have been becoming more frequent in recent decades in India. As a result, people are also experiencing more frequent occurrences of floods in several parts of the country including Chennai. These are considered climate change-induced floods/disasters. But the key question remains. Are we hiding behind climate change for all the blunders made so far? Chennai suffered the serious impacts of floods in 2005, 2015 and, again, in 2023. Although each one of these floods is unique in their own way, the impact has been devastating, increasing year after year. The flood in 2023 is considered the worst in the past 47 years. But the main concern is whether we have learned any lessons from past flood events. Have we resorted to corrective measures?

A few pertinent questions arise in this context: To what extent are the floods that occur due to historical human errors or blunders? To what extent has conventional wisdom followed by the State helped mitigate floods and droughts? What are the lessons learnt from the past extreme events? What should be done to make the city of Chennai flood resilient? And, how does one convert disaster into opportunity so that the city of Chennai also gets abundant water supply?

What is more, the coastal city of Chennai has also got to deal with coastal floods and climate change and the consequent impact of seawater rise. The key issue, therefore, is to highlight the necessity to decode Chennai’s urban and peri-urban hydrology, its ecosystem in its totality, and make meaningful and scientific interventions not only towards flood mitigation but also towards handling droughts and in building climate-resilient strategies for Chennai and Chennai Metropolitan Area (CMA).

Upstream and downstream watersheds

Chennai city and the adjoining districts are richly endowed with wonderful watersheds. There are 3,588 irrigation tanks (some are very large) in Kancheepuram, Chengalpattu and Tiruvallur districts, respectively, as highlighted in tank memoirs. These are man-made but magnificent watersheds created through a series of earthen embankments, constructed across streams which carried heavy flows during the monsoon months. These tanks were constructed in such a way that surplus from an upstream tank served as a feeder to a downstream tank. Unfortunately, these tanks are neglected, silted up with broken bunds and control structures. In addition, catchment areas, flood plains, feeder and supply channels and even the water spread area in many of these tanks are heavily silted and encroached. The net result is a double disadvantage: Water storage in these tanks is very little and run-off is very high (over 80%) which caused heavy damage to the city of Chennai.

In this context, this is what needs to be done: study the urban and peri-urban water dynamics — interconnected hydrological conditions; map water bodies (around 4,000) located in the 5,904 sq.km area of the proposed CMA (spread over Tiruvallur, Chengalpattu, Kancheepuram and parts of Ranipet districts). Since these districts are already substantially urban, there is a dire need to protect these water bodies from encroachments. Not just tanks, but catchment areas, inlet and surplus channels, foreshore areas (tank flood plains) and tank bunds are equally important. The missing links between these tanks need to be restored.

These water bodies need to be restored to their original capacity or, where possible, even double the capacity so that excess water can be saved in these water bodies which will contribute to a substantial saving of run-off water.

A comprehensive hydro-elevation (drainage) mapping needs to be drawn up covering the upstream-downstream watersheds with Chennai and the sea.

Wonderful flood carriers

Chennai is in fact geographically very uniquely placed, which is a blessing. It has three waterways (rivers) that run through the city, something which no other city in the country and in South Asia can boast of. The Kosasthalaiyar river runs through the northern part of Chennai, the Cooum which takes care of central Chennai, the Adyar which caters to southern Chennai, and further south, the Palar which carries the flow. Each of these rivers also feeds numerous tanks before reaching the Bay of Bengal. There is the Buckingham canal which cuts across all the four rivers in close proximity to the sea. Unfortunately, these major drainage systems are in pretty bad shape due to heavy encroachments, more so on the flood plains. These rivers have also lost their gravity and velocity due to sludge and silt deposits. Several efforts have been made to restore these rivers as well as the Buckingham Canal, but the conditions remain far from satisfactory. What these waterways need is year-long attention and maintenance and not cosmetic interventions such as river-front and canal front developments. Besides these major waterways, there are many macro and micro drains such as Okkiam maduvu, Mambalam canal, Velachery canal, Kodungaiyur drain, Otteri nallah, Virugambakkam/Arumbakkam canal, Veerangal Odai, Captain Cotton canal and the Villivakkam canal. These drains as well as the 2,900 kilometre long Storm Water Drain network constructed in the GCC area also deserve year-long attention and maintenance.

Rising urban expansion

At the same time, it is extremely critical to acknowledge the fact that the degree of urban expansion in the Chennai city has been one of the fastest in the country. And, it is important to recognise that the urban expansion process is irreversible and can be disastrous if not regulated. When the city limit was expanded from 174 sq.km to 426 sq.km, and the CMA to 1,189 sq.km, there was very little thought devoted to protecting the ecological hotspots in the expanded areas. In the process, Chennai has lost many water bodies (lakes and ponds) and much of the Pallikaranai marsh land and coastal wetlands. At present, the CMA is to be expanded from the existing 1,189 sq.km to 5,904 sq.km covering the entire districts of Tiruvallur, Chengalpattu, Kancheepuram and parts of Ranipet district as part of the Master Plan III. At least now, will the Chennai Metropolitan Development Authority notify the ecological hot spots and “no development zones” in the III Master Plan so that the existing lakes (around 4,000), flood plains, forests, major drains, flood buffers and wetlands can be protected? Otherwise, one will have to continue hiding behind climate change for all accumulated blunders.

Why was Chennai so badly flooded?

The final word. Chennai city and the CMA can be permanently saved from floods while, at the same time, get round the clock water supply even in a drought year provided the measures indicated above are followed truthfully and scientifically. This is what is called converting disaster into an opportunity.

S. Janakarajan is former Professor, Madras Institute of Development Studies (MIDS) and President, South Asia Consortium for Interdisciplinary Water Resources Studies (SaciWATERs)

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